The invention relates to the detection of exhaust from an internal combustion engine.
The invention arose during development efforts directed toward reducing downtime of large, stationary industrial internal combustion engines continuously operated over long intervals. Such engines generate up to thousands of horsepower, and are used in large scale electrical and motive power generation applications, for example utility company power generation, mining and pumping applications, ocean going vessels, and so on. These engines are characterized by extremely long service intervals, as compared to automotive applications. For example, some of such engines have service intervals longer than the total operational life of automobile engines.
During the noted long intervals between service on large industrial engines, it is desirable to allow continuous operation, without downtime. Furthermore, the engine should operate within specified tolerances during the entire length of such interval, without drifting from allowable specifications. One of such specifications is that the proper air/fuel ratio be maintained within an allowable tolerance window. Another specification is that exhaust emissions be maintained below a given limit.
The noted large, industrial, long interval engines may be provided with an oxygen sensor, for example U.S. Pat. Nos. 4,638,783 and 5,243,954, incorporated herein by reference. The oxygen sensor detects the relative presence of oxygen in the exhaust of the engine and generates an output voltage signal which is fed back to a controller controlling the fuel delivery system to ensure that the proper air/fuel ratio is being supplied to the engine. Some industrial engines, including some lean-burn engines, may be equipped with a catalytic converter. In such applications, the oxygen sensor additionally ensures that the proper exhaust gas constituents are transmitted to the catalytic converter for oxidation and reduction.
There are several types of oxygen sensors. Oxygen sensors were first developed for automotive applications to be used in conjunction with catalytic converters. The motive was to control the mixture of exhaust constituents into the catalyst so that it could do its job, i.e. so that both the oxidation and reduction reactions go to completion. Most automotive applications are based on stoichiometric engines, i.e. engines that run at the chemically correct air/fuel ratio so that the oxygen content remaining after combustion is near zero. One characteristic of stoichiometric engines is a relatively high exhaust temperature, e.g. about 1200.degree.-1300.degree. F. The oxygen sensor is immersed in this high temperature exhaust gas flow, and the exhaust gas heats the sensor to operating temperature. There is a warm-up period, when the car is first started, during which the oxygen sensor is below the temperature range required to operate correctly. During warm-up, the catalytic converter cannot function properly, and exhaust pollutants emitted to the atmosphere are high.
The noted warm-up period spawned the development of oxygen sensors with internal electrical heaters. Legislation addressing automotive start-up emissions was met with a system based on an electrically heated oxygen sensor. These sensors, like their predecessors, are inserted directly into the exhaust stream and give the same type of output, FIG. 1, with a knee or fall-off from a high output to a low output at about zero exhaust oxygen concentration.
As engine designers began searching for different ways to lower exhaust emissions and improve fuel economy, lean-burn technology began to evolve. This technique involves deliberately having excess air in the combustion chamber when the fuel is burned. Lower emissions and better fuel economy are enabled. However, stoichiometric oxygen sensors do not provide a meaningful signal at lean air/fuel ratios, FIG. 1. Thus, if engines were to be properly controlled, a new type of oxygen sensor had to be developed. This was the impetus for lean-burn oxygen sensors.
Lean-burn oxygen sensors have a different output characteristic than stoichiometric oxygen sensors, FIG. 1, and provide a meaningful signal at lean air/fuel ratios. One type of lean-burn oxygen sensor, as shown in FIG. 1, provides a linear output, with an increasing output signal the greater the exhaust oxygen concentration, including in regions of lean air/fuel ratios.
Lean-burn oxygen sensors have an internal electric heater to raise the temperature of the sensor element into its operating range. This is because lean burn engines run at cooler exhaust temperatures than stoichiometric engines. Lean-burn automotive engines typically run at about a 23 to 1 air/fuel ratio and an exhaust temperature of about 1000.degree.-1100.degree. F. Stoichiometric automotive engines typically run at about a 15 to 1 air/fuel ratio and an exhaust temperature of about 1200.degree.-1300.degree. F. Industrial, long interval lean-burn internal combustion engines run leaner and at lower exhaust temperatures than automobile engines. For example, typical ranges for lean-burn industrial engines are an air/fuel ratio of about 30 to 1 and an exhaust temperature of about 800.degree.-900.degree. F., though these ranges vary depending upon the engine and the type and quality of fuel used.
One known method for controlling the air/fuel ratio in industrial engines is to map each individual engine's performance with an emissions analyzer, for example "Predictive NO.sub.x Emissions Monitoring For Stationary Engines", G. Beshouri, Diesel and Gas Turbine Worldwide, May 1994, pp. 18-20. This is costly, and lengthens the time to market. It would be more desirable to use a lean-burn oxygen sensor, and control the air/fuel ratio according to the sensor's output. Lean-burn oxygen sensors developed to date have been directed toward automotive applications, and attempts to apply same in large industrial engines having leaner air/fuel ratios and lower exhaust temperatures have not been successful. Despite extensive searching, lean-burn oxygen sensors for industrial lean-burn engines have not been found in the marketplace. An automotive lean-burn oxygen sensor was used on an industrial lean-burn engine, however the sensor repeatedly failed prematurely. Replacement cost is high, including the downtime necessitated thereby. Unless the premature failures can be prevented, and the oxygen sensor made to last the full duration between service intervals, the use of a lean-burn oxygen sensor is not a feasible offering in industrial engine markets. The present invention addresses and solves this problem.
The invention also addresses another problem in exhaust sensing, namely that of sensor contamination and/or poisoning over time, which is particularly significant in long interval industrial engines. Engine exhaust carries many constituents which are detrimental to oxygen sensor life. These constituents can either poison the sensor, i.e. actually penetrate the sensor material and deactivate it, or mask the sensor, i.e. form a coating around the sensor and entomb it. In normal gaseous fueled engines, these constituents are typically due to additives that serve other useful purposes and are not readily eliminated. In addition to such substances, alternative fuel sources, e.g. natural gas, methane from landfills and sewage treatment facilities, etc., carry other contaminants. Furthermore these applications typically have such contaminants in higher concentrations than do normal applications. Further still, alterative fuel applications have even a greater need for air/fuel ratio control because the composition of the fuel can change significantly at the sites, otherwise fuel economy and exhaust emissions will not be optimized. The present invention addresses and solves this need.